Re: Errors with schema migration and logical replication — expected?

Thanks Adrian for all the help. I filed this as bug #15549. I hope this all
helps get logical replication into the "Running" stage.

On Wed, Dec 12, 2018 at 5:06 PM Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com>
wrote:

> On 12/12/18 3:19 PM, Mike Lissner wrote:
> > This sounds *very* plausible. So I think there are a few takeaways:
> >
> > 1. Should the docs mention that additive changes with NOT NULL
> > constraints are bad?
>
> It's not the NOT NULL it's the lack of a DEFAULT. In general a column
> with a NOT NULL and no DEFAULT is going to to bite you sooner or later:)
> At this point I have gathered enough of those bite marks to just make it
> my policy to always provide a DEFAULT for a NOT NULL column.
>
> >
> > 2. Is there a way this could work without completely breaking
> > replication? For example, should Postgresql realize replication can't
> > work in this instance and then stop it until schemas are back in sync,
> > like it does with other incompatible schema changes? That'd be better
> > than failing in this way and is what I'd expect to happen.
>
> Not sure as there is no requirement that a column has a specified
> DEFAULT. This is unlike PK and FK constraint violations where the
> relationship is spelled out. Trying to parse all the possible ways a
> user could get into trouble would require something on the order of an
> AI and I don't see that happening anytime soon.
>
> >
> > 3. Are there other edge cases like this that aren't well documented that
> > we can expect to creep up on us? If so, should we try to spell out
> > exactly *which* additive changes *are* OK?
>
> Not that I know of. By their nature edge cases are rare and often are
> dealt with in the moment and not pushed out to everybody. The only
> solution I know of is pretesting your schema change/replication setup on
> a dev installation.
>
> >
> > This feels like a major "gotcha" to me, and I'm trying to avoid those. I
> > feel like the docs are pretty lacking here and that others will find
> > themselves in similarly bad positions.
>
> Logical replication in core(not the pglogical extension) appeared for
> the first time in version 10. On the crawl/walk/run spectrum it is
> moving from crawl to walk. The docs will take some time to be more
> complete. Just for the record my previous post was sketching out a
> possible scenario not an ironclad answer. If you think the answer is
> plausible and a 'gotcha' I would file a bug:
>
> https://www.postgresql.org/account/login/?next=/account/submitbug/
>
> >
> > Better schema migration docs would surely help, too.
> >
> > Mike
> >
> >
>
>
> --
> Adrian Klaver
> adrian(dot)klaver(at)aklaver(dot)com
>